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BIG PHYSICS, BIG QUESTIONS –

Icy Trojan asteroids boost planet-forming theory

By Kimm Groshong

This artist’s rendering reveals how the 617 Patroclus binary pair may appear

(Image: Keck Observatory/Lynette Cook)

A pair of asteroids that travel in lockstep near Jupiter are more likely to be dirt-covered ice balls than rocky rubble piles, a new study suggests. The results back a new theory of how the giant planets formed.

The asteroid pair is called 617 Patroclus and belongs to the Trojan asteroids around Jupiter. The duo may share a common lineage with far-flung objects that today orbit beyond Neptune, suggests a calculation of the objects’ densities.

More than one theory exists to explain how the Trojans arrived in their current location. The traditional theory says they formed near Jupiter while the planet itself was coming together. In that case, they would be expected to have a rock and ice composition similar to the Jovian moons Callisto and Ganymede.

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But the new results are consistent with a theory put forward in May 2005 by an international team of researchers. The theory suggests that Jupiter, Saturn, Uranus and Neptune originally formed in a once-densely-filled area close to the Sun, in a great disc of innumerable small bodies, called planetesimals. This circling disc can be likened to a kind of primordial Kuiper Belt – the mysterious zone of objects which now lies beyond Neptune.

According to the group’s computer simulations, it was not until 650 million years after the planets formed that they arrived in their current orbits, having undergone a chaotic migration outwards, flinging planetesimals as they went (Nature vol 435, p 462). Some of these icy comets, they suggested, were captured by Jupiter as Trojans rather than migrating to the Kuiper Belt.

Now the team led by Franck Marchis at the University of California in Berkeley, US, says there is more reason to believe that cosmic story.

“Fairy tale”

The researchers, also from the Observatoire de Paris, France, and the Keck Observatory in Hawaii, US, used a Keck Telescope fitted with special optics to observe the near-circular orbit of Patroclus.

Tracing the orbit and using thermal data, they calculated an “extremely low” density and concluded the two asteroids are largely composed of water ice – backing the idea they were once icy comets, similar to Kuiper Belt Objects.

Marchis still refers to the theory as “a fairy tale” but says he hopes to find more study targets – among roughly 1700 Trojan asteroids – to help uncover the truth. “What we need to do is find more Trojans, more binaries,” he told New Scientist. “Then we can find out if low density is a characteristic of all Trojans.”

Heart-warming

Binary asteroids are important because the pairs allow scientists to calculate the size, mass and density of the asteroids based on how each affects the other’s orbit. Without such closely bound objects, scientists would need spacecraft to fly past the asteroids and take a look.

Hal Levison at the Southwest Research Institute, Colorado, US, and one of the researchers who suggested the new theory says he is pleased to hear that a new measurement seems to support his theory. “This warms my heart,” he told New Scientist . His team’s model successfully accounted for all previous observations when it was created. He adds&colon; “To have something new come in and say ‘Look, that matches too’ is really cool.”

Asteroid expert William Bottke at the Southwest Research Institute in Boulder, Colorado, US, notes that one density measurement does not prove a theory. “But at least it’s consistent with the idea that these bodies could be very similar to Kuiper Belt Objects,” he says.

If further observations back the new hypothesis, Bottke notes it could save a good deal of time and money&colon; “If you wanted a mission to go to a Kuiper Belt Object, one way to do it would be to just go to the Trojans instead. They may represent this population of material that formed just outside of Neptune.”